This may come as a surprise to some as it did to me but there is no braking system. Should there be a need to slow the mill down then the cap would be rotated out of facing directly into the wind and then the cane crushing rollers would be loaded up to slow the mill down eventually with the cane causing the mill to stop. The sails can then be turned backwards manually using ropes secured to the ends of the sails, the rolls cleared and then the sails tied off and secured to horizontal posts built into the lower part of the mill wall.
This is not exactly a very safe system of working and at first sight this would certainly not meet today's health and safety at work (OSHWA ?) standards but a delicate balance of weight and force (torque) is going on in the cap and the introduction of braking forces might well result in unexpected consequences.